The present invention relates to a method of preparing an emulsion- or asphalt-concrete for use as a road material.
It is-a-well known that asphalt concrete is laid out in a smoking hot condition, whereby it is possible to keep the mixture of stones and bitumen suitably shapable for an even laying-out and compression, just as the mixture may then have a large content of high viscid bitumen, thereby ensuring a good bonding and yet a certain, desired resiliency in the laid-out material, when its temperature decreases to ambient temperature. However it is very energy consuming to effect the associated heating of the material.
Against this background has been developed a `cold` technique, which may well have certain drawbacks or limitations, but nevertheless is considered advantageous in that the heating can be avoided. The technique is based on the use of a bitumen emulsion in admixture with a graded stone material. An aqueous bitumen emulsion has no particular bonding ability of its own, but in time a so-called `breaking` of the emulsion occurs, whereby the emulgated bitumen fractions float together and the water is segregated, such that the bitumen may thereafter act as a binding agent that can stick to the stone surfaces and bond these together. By means of different additives it is possible to control rather accurately when this breaking should take place after the mixing operation. It is achievable, therefore, that the mixture can be prepared and transported to the laying area and be laid-out therein prior to the breaking having proceeded to the point where the material will not thereafter be suitably easily shapeable.
If the emulsion is or has not broken almost as soon as it has been laid out, one problem among others will be that in case of rain after the laying out a more or less extensive washing out of the emulsion may take place, this of course being highly unlucky or in the worst case even fatal for the work.
It is well known, therefore, that the breaking should be adjusted so as to have proceeded widely already at the time of the laying out, even though this will create the problem that it is not possible to use any particularly hard binding bitumen in the emulsion. If the breaking of an emulsion with such a hard or high viscid bitumen has proceeded widely just before the laying out, it will be impossible to effect the laying work in an easy and orderly manner under `cold` conditions, because the material will then be so strongly bonded together that it cannot be reasonably easily shapable.
It has become a common practice that for usual applications of emulsion concrete it is only possible to use emulsions based on a relatively low viscid bitumen having a viscosity of up to some 3000 mm.sup.2 /sec at 60.degree. C. However, such materials will have a relatively poor stability, so the method is used only on roads with a low traffic load.
It could be possible to use a bitumen of a higher viscosity and even an increased amount of emulsion if care is taken that the emulsion breaks only partly during the mixing process or prior to the laying out, but this would also create serious problems. Depending on the grain curve of the stone material it may be impossible to secure an optimal bitumen percentage, because the stone material can only `carry` a certain amount of unbroken emulsion without surplus emulsion flowing off. Such a flow off of incompletely broken emulsion will take place during the truck transport of the material from the mixing place to the working place and thus give rise to considerable smudging problems for other road-users. Moreover, after its laying out and compaction on the road, the material will exhibit cavities holding unbroken emulsion that will cause the finished road layer, during a long period of time, to be very sensitive to rain, which causes a washing out of bitumen.
It has earlier been recognized that with a minimized, yet sufficient total amount of emulsion the problem may occur that the emulsion is predominantly `absorbed` by the finer stone fractions, such that a required total wrapping of the coarse stones will not be achieved unless still more emulsion is added. It has been proposed to remedy this by initially supplying to the mixing stage the coarse stone fraction and the amount of emulsion necessary for a total wrapping of these stones, while the finer stone fraction is added later on, if required together with more emulsion, see e.g. U.S. Ser. No. 923,891 and GB-C-334,588. It is possible to thereby avoid a direct waste of surplus emulsion, but there will be no resulting quality improvement of the laid out material.